The critical sensor: a new type of evanescent wave immunosensor

A new planar waveguide immunosensor has been developed in which adsorption at a surface, changing the refractive index contrast, is measured. In this ¿critical¿ sensor the change in the effective refractive index contrast is transducted to a shift of the critical reflection angle. The sensor's response after a specific binding of antigens to antibodies is discussed theoretically. In addition, an experimental sensitivity evaluation on the basis of several immunosensing experiments is presented. The obtained lower detection limit is 2 × 10¿2 nm in adlayer growth, equivalent to 12 pg/mm2 of analyte coverage. This sensitivity is comparable to the performance of the surface plasmon resonance sensors or the grating coupler sensors. However, the ¿critical¿ sensor has some advantages. These are mainly the ease of fabrication and adjustment prior to a measurement, and the fact for an experiment no metal layer has to be used.